The phloem of a plant is a vascular tissue that is responsible for distributing the products of photosynthesis, nutrients and hormones to plant tissues and organs. Associated with the phloem are sieve elements and companion cells. Mature sieve cells are enucleate and must rely on physically connected companion cells (via a branched plasmodesmata) to provide many physiological functions. Sieve cells and companion cells together serve to deliver proteins into the phloem. Research has shown that specific mRNA molecules can be found in the plasmodesmata suggesting that there are mechanisms that participate in mRNA transport through the sieve cell-companion cell plasmodesmata connection (Xoconostle-Cazares, B., et al., (1999) Science 283:94–98). Some plant viruses have been shown to be able to establish systemic infections via movement proteins (MP) that have the capacity to interact with the plasmodemata and foster the cell—cell transport of MP and viral nucleic acids. Thus plant viruses have evolved the capacity to utilize existing plant pathways to traffic macromolecules to surrounding cells. Plants appear to have proteins similar to viral movement proteins that function in the transport of nucleic acids from cell to cell. Several plant genes that encode viral movement protein homologs have been identified in rice (elicitor-responsive gene 3, Os-FIERG1 and Os-FIERG2), one has been identified in corn (novel gene) and one has been identified in Cucurbita maxima (CmPP16) (Xoconostle-Cazares, B., et al., (1999) Science 283:94–98). Interestingly, movement of RNA throughout the plant is postulated by some to explain the phenomena of cosuppression. Thus, understanding plant viral movement protein homologs and how they work will provide mechanisms to control cosuppression and provide mechanisms to engineer plant virus resistance.
RNA-directed RNA polymerase (RdRP) is a plant-specific nucleic acid-synthesizing enzyme. Plants (tomato, chinese cabbage, cowpea, cauliflower, tobacco, and cucumber) are the only eukaryotes in which cellular RdRP has demonstrated Schiebel W., et al., (1998) Plant Cell 10:2087–2101) and furthermore, RdRP does not appear to be an RNA-dependent RNA polymerase, an enzyme that mediates viral RNA replication. The origin and biological function of the enzyme however is unknown. Studies on the antiviral state in transgenic plants suggest that RdRP could play a role in post-transcriptional gene silencing. Thus RdRP might play an important regulatory role in gene expression because it can transcribe RNA sequences (from RNA molecules) that could control the synthesis of nucleic acids and their translation into proteins. Understanding the function of RdRP in plants could provide a valuable tool to control gene expression via cosuppression and provide mechanisms to engineer plant virus resistance.